The present invention relates generally to a front converter lens system using a diffractive surface having a lens action based on a diffraction phenomenon, and more specifically to an optical system which is mounted on a subject-side of a master lens system to vary its focal length.
Conventionally, an optical system has been used with a master lens system (generally a phototaking lens) to make its focal length long or short. To make the focal length long, a substantially a focal arrangement of a positive lens and a negative lens is often used. To make the focal length short, a substantially a focal arrangement of a negative lens and positive lens is often used. These arrangements are generally called an afocal front converter lens system, which is very convenient because the focal length of a camera lens, for instance, can be varied by mere attachment of that optical system thereto.
However, currently available a focal front converter lens systems fail to take full advantage of their convenience because they are made up of some glass lenses in order to achieve satisfactory correction for aberrations and, hence, incur weight and cost increases.
A typical afocal front converter lens system is disclosed in JP-A 6-289289. Example 4 therein is directed to an optical system comprising a lens group of positive power located on a subject side thereof and a lens group of negative power located on an image side thereof, each consisting of a doublet, and having a zoom ratio of 1.3. Each lens group is corrected for chromatic aberrations and all lenses are constructed of glass material; satisfactory image quality is obtained with a corrected Petzval sum. On the other hand, Example 1 therein shows an optical system comprising two lens groups, each consisting of one plastic lens. However, this example uses a special material having extremely low dispersion instead of an ordinarily available plastic material (such as acrylic resin or polycarbonate resin) because, with the latter plastic material, it is difficult to make sufficient correction for chromatic aberrations.
JP-A 4-116511 discloses a wide converter lens system having a zoom ratio of 0.8, with a first lens group of negative power located on a subject side thereof and a second lens group of positive power located on an image side thereof, said first lens group consisting of two glass lenses and said second lens group consisting of one glass lens.
It is not preferable to use glass lenses for these prior systems because of cost and weight increases. Weight problems may be solved by use of plastic lenses, from productivity and cost perspectives, however, this is again not preferable because of the need of some special material. Further, any sufficient performance is not achieved by use of ordinarily available resin materials.
An object of the present invention as will hereinafter be explained is to use a diffractive surface, thereby providing an inexpensive yet lightweight converter lens system having improved performance.
For a better understanding of the invention, the lens action of a diffractive surface is here explained. While a conventional lens is based on the refraction of light at a medium interface, the lens action of the diffractive surface is based on the diffraction of light. Now consider the incidence of light on such a diffraction grating as shown in generally in FIG. 1. Emergent light upon diffraction satisfies the following equation (a): EQU sin .theta.-sin .theta.'=m.lambda./d (a)
where .theta. is the angle of incidence, .theta.' is the exit angle, .lambda. is the wavelength of light, d is the pitch of the diffraction grating, and m is the order of diffraction.
Consequently, if the pitch of a ring form of diffraction grating is properly determined according to equation (a), it is then possible to converge the incident light on one point, i.e., impart a lens action to the diffraction grating. Here let r.sub.j and f the radius of a j-th ring on the grating and the focal length of the diffractive surface, respectively. Then, the following equation (b) is satisfied in a region of first approximation: EQU r.sub.j.sup.2 =2.lambda.f (b)
For a diffraction grating, on the other hand, a bright-and-dark ring form of amplitude-modulated type grating, and a phase-modulated type grating with a variable refractive index or optical path length are known. In the amplitude-modulated type, for instance, the diffraction efficiency the ratio between the quantity of incident light and the quantity of the first order diffracted light) is about 6% at most because plural orders of diffracted light are produced. In the phase-modulated type, too, the diffraction efficiency is about 34% at most. If the diffraction grating is modified such that its section is of such saw-toothed shape as depicted in FIG. 2, however, the diffraction efficiency can theoretically be brought up to 100%. Even though actual losses are taken into account, a diffraction efficiency of at least 95% is then obtainable. Such a diffraction grating is called a kinoform. In this case, the height of each tooth is given by EQU h=m.lambda./(n-1) (c)
where h is the height of the tooth, and n is the index of refraction of a substrate material forming the diffractive surface.
As can be predicted from equation (c), a diffraction efficiency of 100% is achievable at only one wavelength. FIG. 3 illustrates a specific wavelength vs. diffraction efficiency relation at 550 nm design wavelength. As the wavelength deviates from the design wavelength, the diffraction efficiency decreases greatly. With decreasing diffraction efficiency, the remnant light exists as unnecessary light. In the case of an optical system used under white light, care should thus be taken of a flare problem due to such unnecessary light.
How to design a diffractive surface is now explained. The diffractive surface may be designed by some known methods. In practicing the present invention, however, it is preferable to make use of an ultra-high index method, according to which the diffractive surface is known to be equivalent to a refractive surface having a very high refractive index at null thickness. At this time, the index of refraction n(.lambda.) at any wavelength is given by EQU n(.lambda.)=1+{n(.lambda..sub.0)-1}.lambda./.lambda..sub.0 (d)
where .lambda. is an arbitrary wavelength, .lambda..sub.0 is a reference wavelength, and n(.lambda..sub.0) is the index of refraction at wavelength .lambda..
The diffractive surface has two important features when used in the form of a lens. The first feature is an aspherical action. If the pitch of a diffraction grating is properly determined as already stated, it is then possible to converge light on one point. The second feature is that dispersion is very large or, in another parlance, a so-called Abbe's number is found to be -3.45 from equation (d). Chromatic aberrations several times as large as those of a conventional glass material are produced with a minus sign or in the opposite direction. It is also found that strong anomalous dispersion is obtained with a low partial dispersion ratio.
An example of applying such a diffractive surface to optical systems used under natural light is known from an article "Hybrid diffractive-refractive lenses and achromats", Appl. Opt. 27, pp. 2960-2971. This prior publication shows an example of calculation in the case where, based on the principle of correction of paraxial chromatic aberration, the diffractive surface is used in combination with a single glass lens to make correction for longitudinal chromatic aberration. Specifically, the publication shows that the plane side of a plano-convex lens is constructed of a diffractive surface to provide an achromatic condition, and refers to remnant secondary spectra. The publication also shows achromatization by use of a diffractive surface and doublet combination.
U.S. Pat. No. 5,543,966 shows an example of achromatization by use of a singlet and diffractive surface combination. This example is applied to a so-called film camera for the purpose of increasing the performance of a phototaking optical system comprising a positive meniscus lens convex on a subject side thereof and a stop by disposing a diffractive surface on an image-side surface of the lens, thereby making correction for chromatic aberrations.
"Diffractive optics at Eastman Kodak Company", SPIE, Vol. 2689, pp. 228-254 shows applications of diffractive surfaces to a variety of optical systems. In particular, the publication shows applications of the diffractive surface to phototaking zoom lens systems for lens shutter cameras, and to inversed Galilean type finder systems.